The present invention relates to optical media and, more particularly, to the fabrication of refractive index patterns such as gratings within optical fibers. A major objective of the invention is to provide for faster fabrication of refractive-index gratings in optical fibers, with less cost, and without significantly reducing the fiber""s mechanical strength.
Gratings in optical fibers are important structures for optical communications. For example, communications systems using wavelength division multiplexing require gratings to separate the various signals traveling through the optical fibers. Fiber gratings are also used to make sensors. Most fiber gratings are presently fabricated by exposing the core of the fiber to a UV light, having a wavelength around 240 nm, that causes a change in the refractive-index of the fiber""s core. However, because the outer polymer coating of most optical fibers is not transparent to 240 nm light, the fiber""s outer polymer coating must be removed before exposing the fiber core to the UV light to form the grating. The fiber must then be recoated to prevent damage to the glass fiber and to preserve the mechanical strength of the fiber. This recoating must be done in a timely manner because exposing the surface of a silica fiber to humidity and dirt can permanently weaken the fiber, and the mechanical strength of the once-exposed fiber can remain decreased even after the silica core is subsequently recovered with a coating material. The choice of recoating material is limited by the requirement that the recoating material must adhere well to silica and may need to form a hermetic seal to the fiber surface. Additionally, removing the fiber""s polymer coating before UV light exposure and subsequently recoating the fiber with a polymer after the UV light exposure is time consuming and expensive.
Recently, new fiber coatings that are transparent to UV light at 257 nm have been used to coat optical fibers. These new fiber coatings make it possible to fabricate fiber gratings without first having to strip the fiber of its coating. However, these fibers with their special coatings have several disadvantages. The transparency of the coating to UV light makes the fibers sensitive to the environment, since undesirable UV light from the environment can now reach the photosensitive fiber core, producing excess optical loss and, in extreme cases, even erasing the fabricated grating. Additionally, these coatings are especially soft and sticky, and can accumulate dust. This dust can adversely affect grating fabrication if the dust absorbs UV light during the fabrication process. Moreover, the polymer coating can also become damaged by excess heat, which can also distort the fiber grating in the fiber core.
An alternative approach for writing gratings in fibers without removing the fiber coating uses the sensitivity of the fiber core to light in the near-UW region of the spectrum, having a wavelength of approximately 330 nm. An advantage to using near-UV light instead of mid-UV light is that the polymer coating of standard optical communication fiber (such as Corning SMF 28, a product of Corning Incorporated, Corning, New York) is somewhat transparent to near-UV light, but is not transparent to mid-UV light having a wavelength of approximately 240 nm. Because standard polymer coatings are transparent to light in this near-UV wavelength region, it becomes possible to fabricate gratings through standard coatings without the use of a special polymer coating. Standard fiber coatings also provide protection to the photosensitive fiber core from mid-UV light having wavelengths in the region of the spectrum where the fiber core has its highest photosensitivity, so that the problem of induced loss and grating erasure by UV light from the environment is reduced. However the problem of degradation of the polymer coating surface from dust and other environmental contaminants remains, because such dust can absorb UV light when the grating is written and distort the light pattern that forms the grating in the fiber core. Special measures to protect the phase mask from contamination with dust and possible exhaust from the polymer during UV exposure may also be required.
What has been needed, and heretofore unavailable is a faster, lower cost method for writing refractive index gratings into optical fiber that avoids the problems of damage or contamination of the coating and subsequent deterioration of the optical path needed to write the grating into the fiber core region. The resulting fiber must have high immunity to erasure or solarization and must retain a significant fraction of its original mechanical strength. The present invention fills this need.
Briefly and in general terms the present invention solves the problems of protecting the photosensitive fiber core from undesirable environmental UV exposure and shielding the optical polymer surface from degradation. This is accomplished by using a fiber having multiple coating layers. This multicoated fiber contains an inner coating layer that is mostly transparent to the writing light, and a xe2x80x9cprotectivexe2x80x9d coating layer over the inner xe2x80x9copticalxe2x80x9d layer to provide mechanical protection for the inner layer from dust and other contaminants, and optical protection of the photosensitive fiber core from undesired photo-darkening and solarization. The protective layer is easy to remove without significantly disturbing the inner layer. This removal of the protective layer can be accomplished by either mechanical or chemical means. The protective layer is removed primarily in the region where the optical writing exposure is executed, for example only around the region of the fiber where the grating will be written. Removal of the protective layer should be done immediately before writing with UV light so the outer surface of the inner layer will have no time to degrade or accumulate dust. After writing with UV light the protective layer and any other outer layers are then reformed in the regions where they were removed.
One key advantage of the new technique as compared to the standard stripping and recoating technique is that the silica glass of the fiber itself is never exposed to the environment so that the fiber retains a greater mechanical strength. Exposing the surface of a silica fiber to humidity and dirt can permanently weaken the fiber, and the mechanical strength of the once-exposed fiber can remain decreased even after the silica core is subsequently recovered with a coating material. Another advantage of the new technique is that the protective polymer does not necessarily have to adhere well to silica; the recoating process does not require hermetic sealing of the outermost coating to the fiber surface. The less stringent requirements for stripping and recoating simplify these processes so that time and expense can be saved.
One embodiment of the invention comprises a waveguide for processing with actinic radiation. The waveguide, in the form of an optical fiber, has a coating containing at least an inner layer and a separate protection layer. The inner layer may be at least half as transmissive as absorptive for actinic radiation, where the actinic radiation may have a wavelength longer than 220 nm although other wavelengths of actinic radiation may also be used. The core of the waveguide is sensitive to actinic radiation. The protective layer of the waveguide is removable either mechanically or chemically and can be removed without removing the inner layer.
The waveguide of this type allows for a method of processing the waveguide so as to affect the sensitive core. The method consists of removing the protective portion of the coating followed by exposure of the core to the actinic radiation through the remainder of the coating. This exposure can be uniform or non uniform along the waveguide. One purpose of exposing the waveguide and its sensitive core to the actinic radiation is to fabricate a grating structure within the waveguide. Many different types of patterns or gratings may be produced within the core of the waveguide. Examples of such gratings include Bragg gratings and long-period gratings. The method can be used to produce long period gratings covering a range of average periods in the range of 10 to 2000 microns.
The method also includes the recoating of the waveguide with a protective layer after its exposure to actinic radiation. This protective layer should be at least half as absorptive as transmissive for the actinic radiation. The protective layer is typically a polymer, which may be a polymethacrylate, silicone resin or any other appropriate material including, aliphatic polyacrylates, silesesquioxanes, alkyl-substituted silicones or vinyl ethers.
The actinic radiation is typically Ultra-Violet radiation in the region 220-390 nm, although other wavelengths of actinic radiation may also be used. The method can also include preloading the waveguide before exposure to UV light with hydrogen (or its isotope deuterium) to increase the photosensitivity of the core to the UW light.
Another embodiment of the invention includes a refractive index grating formed within the core region of an optical fiber by exposure of the fiber core to actinic radiation. The grating containing fiber has a mechanical breaking strength of not less than 50% of the same fiber prior to processing the fiber to fabricate the grating. It is anticipated that the breaking strength could actually be 90% or more of the strength of the original fiber. The optical fiber used in fabrication of the grating is the same as previously discussed, having an inner layer and a separate protective layer, with the protective layer being removed prior to creation of the grating, then replaced with a new protective layer in the region containing the grating.
Other features and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings, which illustrate, by way of example, the features of the invention.